Power amplifiers (PAs) are important units in various communication systems. There are usually two ways to improve performance of a PA. One is to increase an efficiency of the PA, and the other is to improve heat dissipation of the PA.
A power transistor, as a core device of a PA, produces the majority of heat in the PA. In general, the higher the power of a power transistor is, the more the power transistor produces heat. Therefore, for a high power transistor contained in a PA, heat dissipation thereof has to be carefully considered for performance of the PA.
Heat dissipation methods for a high power transistor such as a high power Radio Frequency (RF) transistor used in a radio communication base station have been proposed in the prior art. How to fix the high power transistor to a PCB is a main concern of the heat dissipation methods.
Currently there are two kinds of solutions to fix a power transistor to a PCB. One kind of solution is a manual fixing solution in which for example screws are used to fix a flange of a power transistor onto a heat sink, which is called a bolt down technique. The bolt down technique is generally used for a high power RF transistor with a ceramic package and a RF output power of more than 20 W. Manual work of an operator is generally needed for implementing the bolt down technique. Hence, the technique greatly depends on the operator's experience, resulting in a low working efficiency, a low consistency, and a low stability.
The other kind of solution is a Surface Mounted Technique (SMT) solution in which pins and a flange of a power transistor are soldered on pads of a PCB. The SMT solution comprises two types of techniques. One type of technique is a through-via technique where a pad for soldering to the flange of the power transistor has some plugged through vias, which is schematically shown in FIG. 1. Referring to FIG. 1, there are a lot of plated through vias on a pad of a PCB under a power transistor. These through vias are plugged with an epoxy, a metal, or any other suitable material. The through-via technique has a limited ability to dissipate heat, so it is only applicable to a low power transistor, e.g. a transistor with a power of no more than 20 W. Moreover, although the ability to dissipate heat will be improved if these through vias are plugged with a metal or copper plated thereon is thickened, a cost of the PCB will have a significant increase, for example, an increase of about 30 percent.
The other type of technique in the SMT solution is an attached coin technique where a pad for soldering to the flange of the power transistor has an attached coin. In the attached coin technique a PCB is produced with a cutout. According to a shape of the cutout, the attached coin technique is divided into a straight through cutout technique, a stepped coin technique, and an embedded coin technique.
There are two structures for the straight through cutout technique. One is called through rout, and the other is called stamped coin and pedestal, which are schematically illustrated in FIGS. 2A and 2B, respectively. For the through rout, a high temperature solder or a conductive paste is used to attach a coin to a PCB, and then a RF power transistor is soldered to the coin and the PCB together with other components. For the stamped coin and pedestal, a coin and a PCB are fixed on a pedestal with a high temperature solder or a conductive glue. Then the whole PCB with the coin and the pedestal goes through reflow for soldering a surface mounted RF power transistor and other components.
FIG. 3A schematically illustrates a configuration of a PCB having a RF power transistor soldered thereon through the stepped coin technique. To implement the stepped coin technique, different layers in the PCB have different cutout sizes. For example, as shown in FIG. 3B, for a four-layer PCB, a cutout on the two upper layers 1 and 2 has a size of 10 mm*20 mm, and the cutout on the two lower layers 3 and 4 has a size of 15 mm*30 mm. So the cutout looks like a step on the PCB from a side view. PCB manufacturers generally handle this kind of PCB with two ways. One way is to make cutouts with different sizes in layers 1, 2 and layers 3, 4. Then a core and a prepreg are stacked, and the PCB is produced in a normal process. The other way is to make cutouts with the same size in all of layers 1, 2, 3 and 4 and produce the PCB in a normal process. Then special cutting tools are used to enlarge the cutout on layers 3 and 4.
In the configuration of the PCB as shown in FIG. 3A, a metal coin is soldered on the stepped cutout of the PCB by a sweat bonding process. Pins and a flange of the RF power transistor are respectively soldered on the PCB and on the metal coin at the same time through an SMT line.
FIG. 4 schematically illustrates a configuration of a PCB having a RF power transistor soldered thereon through the embedded coin technique. As shown in FIG. 4, a coin has two “wings”. A top and a bottom surface of the coin have the same size as a flange of the RF power transistor. A size of a cutout in middle layers of the PCB is larger than that of the cutout in a top and a bottom layer thereof, so the cutout has concave recesses as shown, which can fix the two “wings” of the coin. A process for implementing the embedded coin technique is very complex. Separate cores and prepregs should be cut out with required sizes and then the coin is embedded before PCB pressure.
The attached coin technique comprising the various techniques as stated above has a number of drawbacks. One of the drawbacks is that the attached coin technique is more complex than the through-via technique, resulting in a higher manufacture cost. For example, it is necessary for the straight through cutout technique and the stepped coin technique to solder or paste a coin to a PCB from a bottom side of the PCB before a normal reflow process. The embedded coin technique is the most expensive in the attached coin technique, which increases a PCB cost by over 50 percent. Moreover, only one PCB supplier can provide the embedded coin technique, hence leading to a problem of a single source.
Another drawback is that the attached coin technique has higher requirements on design and PCB manufacturer, because RF performance is significantly impacted by mechanical precision, and quality of connection between the coin and the PCB. Without good connection quality and tolerance controlling, the coin has a risk to fall down when going through reflow. Bad soldering quality and mechanical tolerance can reduce the RF performance.
A further drawback is that the attached coin technique has many application limits as follows:                To use the attached coin technique, a thickness of the PCB should meet minimum requirement in order to keep less distortion and provide good adsorption affinity to the coin under the PCB. That is, the thickness of the PCB can not be less than a certain value.        The coin size can not be as small as desired in view of produce technique and reliability. This means there may be some redundance for the coin size.        For the through rout in the straight through cutout technique, RF reference ground plane is only on the bottom layer, which means the PCB can not be in any thickness in view of RF ground loop.        In the embedded coin technique, the coin should be embedded in the PCB. PCB stack should meet a certain requirement to embed the coin therein. For a multilayer PCB, if the number of the layers is above 10, it is hard to implement the embedded coin technique.        